Noble metals such as Pt (platinum) and Rh (rhodium) show high catalytic activities and have been widely used as catalytic components of three-way catalysts that can simultaneously clean up carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) contained in emissions.
Of these noble metals, Rh satisfactorily reduces NOx. Thus, for improving the thermostability, Rh is supported by perovskite-type composite oxides represented by the general formula ABO3 through impregnation, for example.
Pt also exhibits insufficient thermostability, although it satisfactorily oxidizes CO even at low temperatures. Thus, for improving the thermostability, Pt is supported by perovskite-type composite oxide represented by the general formula ABO3 through impregnation, for example.
Additionally, by allowing Rh and/or Pt to coordinate in perovskite-type composite oxides, the thermostability can be more improved and the exhaust gas purifying capability can be more increased than by supporting Rh and/or Pt through impregnation.
For example, La0.8Ba0.2Ni0.48CO0.50Rh0.02O3, La0.2Ba0.7Y0.1Cu0.48Cr0.48Pt0.04O3, La0.9Ce0.1Co0.9Pt0.05Ru0.05O3 (Japanese Unexamined Patent Publication No. 8-217461), La0.4Sr0.6Co0.95Rh0.05O3 and La0.4Sr0.6CO0.95Pt0.05O3 (Japanese Unexamined Patent Publication No. 5-76762) have been proposed as perovskite-type composite oxides to which Rh and/or Pt coordinate.
Further, a perovskite-type composite oxide with a specific composition of LaFe0.57CO0.38Pd0.05O3 suppresses grain growth and maintains high catalytic activity over a long time. This is because of a self-regenerative function of the perovskite-type composite oxide, in which Pd is reversibly introduced or extracted to and from a perovskite-type crystal structure corresponding to oxidation-reduction change of emissions. These findings have been obtained in recent years (Y. Nishihata et al., Nature, Vol. 418, No. 6894, pp. 164-167, 11 Jul. 2002).
However, in the perovskite-type composite oxides described in Japanese Unexamined Patent Publication No. 8-217461 and Japanese Unexamined Patent Publication No. 5-76762, it is difficult to prevent grain growth by reversibly introducing and extracting Rh and/or Pt to and from a perovskite-type crystal structure corresponding to oxidation-reduction change of emissions, as described in Nature, Vol. 418, No. 6894, pp. 164-167.